Electrical theory 1.7 - Transformers

In this section we'll discuss transformers. As stated before, low voltage lights are safer than mains supply lights (110V+).

• Transformers provide electrical safety by isolating and reducing the voltage from 110 … 240Vac to 12 or 24Vac. You don't need to know how they work - if you want to know see reference http://en.wikipedia.org/wiki/Transformer.
  
  

   

   
• Transformer device may output AC or DC voltage (it has an internal diode bridge and usually a storage capacitor) – check the nameplate as shown in photos. AC symbols: AC, ac, ~      DC symbols: DC, dc, =  .

1. To check if it's a DC device: energise device and measure its DC output with DMM set to 200V DC.
2. To check if it's a DC device with storage capacitor: energise device and measure DC output with DMM set to 200V DC then turn off device; measure output voltage again immediately. If the output voltage stays near the same value, it has a storage capacitor. (Over time this voltage will decay to 0V.)

• The transformer pack shown in LHS picture usually output 10 - 30% higher voltage than rated voltage at no load, eg 10V rating = 13V measured.  The reason is the transformer is undersize (for Watts output) and the manufacturer poorly compensates by increasing the output voltage. This factor must be considered with LED lights which can be greatly affected by supply voltage changes.

• Transformers have a power rating which must exceed the light’s load otherwise it gets hot and burns out.

Here’s the equations:

• Transformer nameplate: 10VA or 24V * 0.42A (approx 10VA)
• LED loading: no. LEDs * power per LED (V*I) eg 60*3.0*.02= 3.6VA  OR
• Bulb loading: no. bulbs * power per bulb (V*I) eg 12*5*.15= 9VA

Result: Either the LED or bulb strings will be suitable for the transformer.

This is all the important info. required.

Note: for lights, the VA value equals the same value in Watts (W).

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